It has long been considered desirable to determine the location of mobile vehicles and other mobile objects. One type of system for determining the location of mobile objects is known as a signpost, or proximity, system. In a signpost system, the location of a mobile object is determined by detecting that the mobile object is near a signpost whose position is known. A common method of detecting that a mobile object is near a signpost is for the signpost to broadcast a signal for the mobile object to receive. The signal can only be received by the mobile object in a broadcast pattern near the signpost.
In many signpost systems the signposts are so far apart that the broadcast patterns of the signposts do not overlap. Knickel in U.S. Pat. No. 3,568,161 teaches one such system. Non-overlapping signpost systems are limited. In such systems, there are gaps between signpost broadcast patterns. A mobile object in a gap between signposts cannot communicate with the signposts. Thus the location of a mobile object in a gap between signposts cannot be directly determined. Also, the situation where the mobile object receiver has failed cannot be distinguished from the situation where the mobile object is in a gap. Also, the signposts cannot serve as reliable repeaters for transmissions from mobile objects. Also, when such systems are being installed, signposts must be carefully placed and the transmitting power of the signposts adjusted so that the broadcast patterns do not overlap.
In other signpost systems, the broadcast patterns of the signposts do overlap. Frenkel in U.S. Pat. No. 3,922,678 teaches one such system. In such systems, each signpost broadcasts a signal that uniquely identifies the signpost. In such systems, the location of the mobile object is determined by detecting that the mobile object is receiving only a single signpost and also by detecting that the mobile object is receiving certain unique combinations of uniquely identified signposts. Such overlapping signpost systems are limited by the difficulty and expense of positioning and adjusting signposts during installation so that overlap areas correspond to predetermined geographic areas.
Most signpost systems are also limited by lack of redundancy. In most systems, if one signpost fails or if the broadcast pattern of a signpost changes, the location of the mobile object cannot always be determined accurately.
Many signpost systems use many communication channels and require frequent transmissions by signposts. It is desireable to minimize the number of communication channels and the number of transmissions by signposts.